contrib/ovs: TypeError: bad operand type for unary -: 'NoneType'

This fixes the following exception:
When Stream.__scs_connecting doesn't change self.state, Stream.connect()
returns None as implicit return value. Thus, the following exception is raised.

conf_switch_set_handler 00003247b5e63145 ovsdb_addr tcp:172.17.60.198:6634
_ovsdb_update 00003247b5e63145 tcp:172.17.60.198:6634 172.17.60.198
_add_tunnel_ports <ryu.app.tunnel_port_updater.TunnelDP object at 0x14a5950> set([])
172.17.60.198 - - [2012-11-21 15:01:00] "PUT /v1.0/conf/switches/00003247b5e63145/ovsdb_addr HTTP/1.1" 201 120 0.267351
remote tcp:172.17.60.198:6634
Traceback (most recent call last):
  File "/usr/lib/pymodules/python2.7/gevent/greenlet.py", line 390, in run
    result = self._run(*self.args, **self.kwargs)
  File "/home/yamahata/openvswitch/sp-lab/scarab/gre-tunnel/ryu-2/bin/ryu/app/tunnel_port_updater.py", line 289, in _serve_loop
    self._init()
  File "/home/yamahata/openvswitch/sp-lab/scarab/gre-tunnel/ryu-2/bin/ryu/app/tunnel_port_updater.py", line 125, in _init
    self.ovs_bridge.init()
  File "/home/yamahata/openvswitch/sp-lab/scarab/gre-tunnel/ryu-2/bin/ryu/lib/ovs/bridge.py", line 103, in init
    self.br_name = self._get_bridge_name()
  File "/home/yamahata/openvswitch/sp-lab/scarab/gre-tunnel/ryu-2/bin/ryu/lib/ovs/bridge.py", line 111, in _get_bridge_name
    self.run_command([command])
  File "/home/yamahata/openvswitch/sp-lab/scarab/gre-tunnel/ryu-2/bin/ryu/lib/ovs/bridge.py", line 99, in run_command
    self.vsctl.run_command(commands, self.timeout, self.exception)
  File "/home/yamahata/openvswitch/sp-lab/scarab/gre-tunnel/ryu-2/bin/ryu/lib/ovs/vsctl.py", line 1006, in run_command
    self._run_command(commands)
  File "/home/yamahata/openvswitch/sp-lab/scarab/gre-tunnel/ryu-2/bin/ryu/lib/ovs/vsctl.py", line 999, in _run_command
    self._do_main(commands)
  File "/home/yamahata/openvswitch/sp-lab/scarab/gre-tunnel/ryu-2/bin/ryu/lib/ovs/vsctl.py", line 918, in _do_main
    self._init_schema_helper()
  File "/home/yamahata/openvswitch/sp-lab/scarab/gre-tunnel/ryu-2/bin/ryu/lib/ovs/vsctl.py", line 805, in _init_schema_helper
    vswitch_idl.OVSREC_DB_NAME)
  File "/home/yamahata/openvswitch/sp-lab/scarab/gre-tunnel/ryu-2/bin/ryu/lib/ovs/vsctl.py", line 793, in _rpc_get_schema_json
    error, reply = rpc.transact_block(request)
  File "/home/yamahata/openvswitch/sp-lab/scarab/gre-tunnel/ryu-2/bin/ryu/contrib/ovs/jsonrpc.py", line 306, in transact_block
    error = self.send(request)
  File "/home/yamahata/openvswitch/sp-lab/scarab/gre-tunnel/ryu-2/bin/ryu/contrib/ovs/jsonrpc.py", line 240, in send
    self.run()
  File "/home/yamahata/openvswitch/sp-lab/scarab/gre-tunnel/ryu-2/bin/ryu/contrib/ovs/jsonrpc.py", line 200, in run
    retval = self.stream.send(self.output)
  File "/home/yamahata/openvswitch/sp-lab/scarab/gre-tunnel/ryu-2/bin/ryu/contrib/ovs/stream.py", line 213, in send
    return -retval
TypeError: bad operand type for unary -: 'NoneType'
<Greenlet at 0x133fd10: <bound method TunnelDP._serve_loop of <ryu.app.tunnel_port_updater.TunnelDP object at 0x14a5950>>> failed with TypeError

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>